In the modern electrolytic industry, soluble electrodes typified by carbon electrodes are being replaced by those electrodes which have electrode active materials. In particular, oxides of platinum group metals, coated on substrates made of titanium or titanium alloys, are very stable under anodic polarization conditions. These electrodes, generally referred to as dimensionally stable anodes (DSA) or dimensionally stable electrodes (DSE), have such superior electrolytic characteristics and durability that they are currently employed in a number of industrial electrolytic processes including the generation of oxygen gas by electrolysis of water, as well as the production of halides and alkali hydroxides by electrolysis of aqueous solutions of metal halides, as disclosed in U.S. Pat. Nos. 3,711,385 and 3,632,498.
Anode materials also play an important role in the production of ozone gas or peroxides utilizing the oxidation reaction involved in anode electrolysis, as well as in other types of electrolysis such as the electrolytic oxidation of organic matter. However, the electrolytic reaction involved in the production of ozone gas and peroxides requires such a high potential that other types of electrolytic reactions which proceed at lower potentials will take place preferentially even if the above-described DSAs are employed. In other words, DSAs are not suitable for the electrolytic production of ozone gas or peroxides with the only exception limited to platinum-coated titanium electrodes.
In view of the need to search for an anode active material that can substitute for DSAs and to improve the efficiency of operations in electrolysis involved in the production of ozone gas and peroxides, various studies have been conducted not only with respect to electrode materials but also in regard to the structure of electrodes and electrolytic cells. Electrode materials of lead, lead oxide and carbon have been studied.
In the anodization processes described above, electrolysis is normally performed with the catholyte and anolyte being separated by a diaphragm in order to prevent reduction from occurring at the cathode. This approach, however, has disadvantages in that a voltage drop occurs due to the electrical resistance of electrolyte present between anode and cathode and that the electrode area cannot be made large enough to ensure high current density. In order to solve these problems, an SPE (solid polymer electrolyte) process has been proposed in which a diaphragm formed of an ion-exchange member is coated with an electrode active material so as to substantially eliminate the Ohmic loss due to the electrolyte.
This SPE process is also applicable to the production of ozone and peroxides by anodic electrolytic oxidation, and an electrolytic apparatus adapted to the SPE process can be fabricated using lead, lead oxide or carbon as an electrode active material. One of the problems associated with the use of an SPE in electrolysis concerns the electrical connection between the current collector and the SPE. The amount of current that can be supplied will increase with the area of contact between the collector and the electrode active material deposited on the membrane. However, collectors are usually porous, and the electrode active material does not adhere very strongly to the membrane. Thus, it is impossible to connect these members over a large contact area without causing Ohmic loss due to the contact between them. Therefore, it is of great importance to find an efficient way for supplying power to the SPE.
Problems also occur if the conventional process for the fabrication of SPEs is directly applied to the production of SPEs using lead, lead dioxide or carbon as an electrode active material. In order to ensure that fine particles are firmly adhered to an ion exchange membrane by hot pressing, temperatures on the order of 350.degree. C. are necessary but partial decomposition of lead dioxide might occur at these temperatures. A method is also known that involves electrodeposition of the particles of an electrode active material. However, this method has disadvantages in that it is difficult to obtain an adequate thickness of coating and that the electrode active material gets into the bulk of an ion-exchange membrane unevenly thus potentially causing side reactions. A technique based on electroless plating is also defective in that control of the plating process is difficult and that an unduly long time is required to perform plating.
With a view to improving the above-described methods of SPE production, it has been proposed that a sintered titanium substrate be coated with lead dioxide to form an electrode, which then is adhered closely to an ion-exchange membrane [J. Elec. Chem. Soc., 132 (1985), P. 367 ff.]. This method, however, lacks production efficiency with respect to the difficulty in forming a titanium sinter. U.S. Pat. No. 4,416,747 proposes a way to solve this problem by forming a layer of fine lead dioxide particles and an organic binder on the surface of a cation-exchange membrane. This approach is effective if an appropriate organic binder is chosen but insufficient activity of the electrode active material is observed because the binder will mask the fine particles of lead dioxide which serve as the active material. Furthermore, it is considerably difficult in practice to select a binder having good adhesion and high durability.